A conventional technique of coating hydrogel capsules is to construct a polylysine membrane around the capsule. See F. Lim, U.S. Pat. No. 4,352,883. Other techniques involve methylcellulose compounds, polyvinyl alcohols, plastics, and other compounds. See L. Lachman, H. A. Lieberman and J. L. Kanig, eds. "The Theory and Practice of Industrial Pharmacy," Lea and Febiger, Philadelphia, PA, 1970, p. 197-225.
However, conventional membranes do not control the passage of solvents, particularly water. An additional drawback is the failure to control the passage of small molecular weight solutes across the membrane, unless the membrane is rigid and virtually impermeable.
The above limitations are important when the capsule to be coated is a hydrogel capsule intended to contain living material, notably botanic tissue, such as seeds, somatic embryos and other meristematic tissue. In these applications the membrane must allow respiration by the tissue contained in the capsule, thus an impermeable capsule is undesirable. Yet it is important that the capsule retains sufficient moisture and solutes for tissue viability. Furthermore, the capsule and membrane are often desirably a temporary container for encapsulated tissue, and they will deteriorate or fracture at the desired time for emergence of the tissue, such as e.g. germination.
In addition to the above limitations with available capsule coatings, in some applications it is desirable to provide a coated capsule which is not cohesive or adhesive, as this facilitates handling and bulk storage.